[Feat] Parse functions

[anhgelus]

Very naive simple implementation of parsing custom functions.

Create a new function with gomath.NewFunction(expr string) where expr follows this syntax:

arg1, arg2, arg3... -> expression

where expression is a valid math expression. It returns a new gomath.Function which can be valuated with a map describing every arg value to return a gomath.Result. It also returns the number of arguments.